The present invention relates to silicone compositions suitable for forming pressure sensitive adhesive compositions. More particularly, the present invention relates to solventless or high solids-containing, addition-curable silicone compositions suitable for forming pressure sensitive adhesive compositions having excellent tack and adhesive properties.
The term "pressure sensitive adhesive" (PSA) as used herein refers to adhesives that can be adhered to a surface and yet can be stripped from the surface without transferring more than trace quantities of adhesive to the surface, and can be readhered to the same or another surface because the adhesive retains some or all of its tack and adhesive strength. Silicone pressure sensitive adhesives have excellent adhesive and cohesive strength and high tack as well as excellent heat resistance, cold resistance, electrical properties, and the like, which makes them applicable for use as electrical-insulating tape and for various pressure-sensitive products which must be resistant to hot and cold.
However, a drawback associated with silicone pressure sensitive adhesives is the use, handling, and emission of flammable and volatile organic compounds, e.g., organic solvents, in the preparation of the pressure sensitive adhesives from compositions containing high levels of organic solvent. Conventional pressure sensitive adhesives are made using high viscosity polymer inputs. Solvent is generally used for the purpose of reducing the viscosity of the composition to a level which renders the curable composition processable. It would be desirable to provide solventless or high solids containing (i.e., low solvent-containing) pressure sensitive adhesive silicone compositions containing low viscosity polymer inputs.
Another drawback associated with conventional pressure sensitive adhesives is their requirement for high cure temperature, e.g., 165.degree. C. and higher, in order to cure effectively. Because such pressure sensitive adhesives require high temperatures to cure, they cannot be used on substrates such as polyolefin-backed substrates which are sensitive to high temperatures.
Addition-curable, low solvent-containing silicone compositions capable of curing to form pressure sensitive adhesive compositons are known in the art.
U.S. Pat. No. 3,983,298 (Hahn et al.) is directed to a silicone composition suitable for use as a pressure sensitive adhesive and obtained by mixing components consisting essentially of (a) 50-60 parts by weight of a solid, benzene-soluble resin copolymer consisting essentially of R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, which is known as an "MQ" resin, (b) 40-50 parts by weight of a vinyl-terminated polydiorgano-siloxane having a viscosity of from 20,000 to 100,000 centipoises at 25.degree. C., (c) a hydrogen-containing organopolysiloxane in an amount sufficient to provide from 1.0 to 20.0 silicon-bonded hydrogen atoms for every olefinically unsaturated radical in the total of (a) plus (b), and (d) a platinum-containing catalyst. It is pointed out in Hahn et al. that compositions of the prior art based on mixtures of a benzene soluble resin copolymer containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units (hereinafter sometimes referred to as "MQ resin") and low viscosity silicones do not form pressure sensitive adhesive compositions.
U.S. Pat. No. 4,774,297 (Murakami et al.) is directed to a composition suitable for forming a pressure sensitive adhesive having excellent tack and adhesive strength, comprising (A) 30-70 parts by weight of a vinyl-terminated polydiorganosiloxane having a viscosity of at least 500,000 centipoises at 25.degree. C., (B) 70-30 parts by weight of an organopolysiloxane containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, (C) an organohydrogensiloxane in an amount sufficient to provide 1-20 silicon-bonded hydrogen atoms per alkenyl group, (D) a platinum-containing catalyst, and (E) from 25-400 parts by weight of an organic solvent. The vinyl polymer used in the Murakami et al. composition has a viscosity of at least 500,000 centipoises and preferably at least one million centipoises at 25.degree. C.
European Patent Application No. 0269454 (Murakami et al.) discloses a composition suitable for forming a pressure sensitive adhesive having excellent tack and adhesive strength and comprising an alkenyl group-containing silicone polymer, a tackifying silicone resin, an organohydrogensiloxane, and a platinum-containing catalyst. There is said to be no specific restriction on the molecular weight of the alkenyl group-containing silicone polymer as long as no adverse effect accrues with respect to the workability of the pressure sensitive adhesive composition. If the pressure sensitive adhesive composition is solventless, the viscosity of the alkenyl group-containing silicone polymer is no more than 100,000 centipoises at 25.degree. C., while in a solvent-containing composition, the alkenyl group-containing silicone polymer should have a viscosity of at least one million centipoises at 25.degree. C. The organohydrogenpolysiloxane should be present in an amount sufficient to provide 1 to 40 silicon-bonded hydrogen atoms per alkenyl group in the composition. The Murakami et al. reference does not disclose a pressure sensitive adhesive composition prepared with low viscosity vinyl-functional silicones.
U.S. Pat. No. 4,988,779 (Medford et al.) discloses a composition having a solvent content of no more than 5-10% by weight and suitable for forming a pressure sensitive adhesive, wherein the composition comprises from 30 to 50 parts of a vinyl endblocked polydiorganosiloxane fluid having a viscosity of from 500 to 10,000 centipoises at 25.degree. C., from 50 to 70 parts of a benzene soluble resin copolymer containing R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units, an organopolysiloxane having silicon bonded hydrogen atoms, and a platinum catalyst. The hydrogen-containing organopolysiloxane is present in an amount sufficient to provide from 1.0 to 30.0 silicon bonded hydrogen atoms for every olefinically unsaturated radical in the composition.
European Patent Application No. 0355991 (Boardman) is directed to a pressure sensitive adhesive composition having a high solids content, i.e., typically in excess of 95% by weight, preferably in excess of 98% by weight, comprising (a) an MQ resin containing silicon-bonded hydroxyl radicals ranging from 1 to 4 percent by weight of the copolymer, (b) an alkenyl-terminated polydiorganosiloxane, (c) a hydrogen-terminated polydiorganosiloxane, (d) a crosslinking agent, and (e) a hydrosilation catalyst. The number of repeating units in the alkenyl-terminated polydiorganosiloxane and the hydrogen-terminated polydiorganosiloxane ranges from 0 to 1000 in each. The crosslinking agent is an organosiloxane containing more than two groups that will condense with a hydrogen or alkenyl group of an organopolysiloxane in a hydrosilation reaction. The ratio of the total number of silicon-bonded alkenyl groups present in (d) and (b) to the total number of silicon-bonded hydrogen atoms present in (d) and (c) ranges from 0.8 to 1.2, i.e., the ratio of the total number of silicon-bonded hydrogen atoms in (d) and (c) to the total number of silicon-bonded alkenyl groups in (d) and (b) ranges from 0.83-1.25:1. The results reported in the examples show that the tack obtained therein ranged from low to moderate. The Boardman composition contains an external crosslinking agent, the examples therein using 1,3,5,7-tetravinyl-tetramethylcyclotetrasiloxane as the crosslinker. It is desirable to provide a solventless or high solids-containing addition-curable pressure sensitive adhesive composition which does not require the presence of an external crosslinker.
It is further desirable to provide solventless or high solids-containing pressure sensitive adhesive compositions containing polymer inputs having viscosities lower than those taught in the prior art. It is also desirable to provide solventless or high solids containing pressure sensitive adhesive compositions which cure to form pressure sensitive adhesives having high tack values as well as high peel adhesion values. It is further desirable to provide solventless or high solids content, addition-curable silicone pressure sensitive adhesive compositions which can be cured effectively at relatively low temperatures to form pressure sensitive adhesives having high tack and peel adhesion values.
Such silicone pressure sensitive adhesive compositions are provided by the present invention.